While urinary tract obstruction is a major cause of renal insufficiency at ages, the long-term renal consequences of urinary tract obstruction are more severe in the developing kidney than in the mature kidney. Following unilateral ureteral obstruction (UUO), there is marked activation of the intrarenal renin-angiotensin system, and heterogeneity in filtration among individual nephrons. The long-term renal consequences of chronic unilateral ureteral obstruction (UUO) are dependent on the balance between two nephron populations: those undergoing progressive atrophy, and those that maintain structural integrity and function. Renal tubular epithelial (RTE) cells undergo apoptosis or necrosis, leading to tubular atrophy. This is associated with progressive interstitial fibrosis and glomerular sclerosis. The aim of this project is to identify the intercellular signaling mechanisms underlying the nephron response to UUO. The following hypotheses will be tested: 1. the regional vascular response to UUO is modulated by the transmission of signals from the renal tubular epithelial cells (RTE) cells to vascular smooth muscle and endothelial cells; 2. apoptosis of RTE cells in response to UUO is modulated by interstitial macrophages attracted to dilated tubules by signals from RTE cells and capillary endothelial cells; 3. Interstitial fibrosis is mediated by cytokine signaling to fibroblasts and macrophages. These hypotheses will be tested by examining the renal cellular responses to UUO in neonatal and adult rats and mice, as well as cell culture of RTE cells, macrophages, and fibroblasts. Mutant mice with GFP expressed by the renin promoter and microdissection of rats will be used to track altered renal microvascular renin expression resulting from UUO. The role of vascular endothelial growth factor (VEGF) and its receptors will be studied in rats with UUO. The role of integrins in the renal response to UUO will be examined in beta2 integrin deficient mice. The cytokines responsible for communication between RTE cells and macrophages and between RTE cells and interstitial fibroblasts will be examined in an in vitro model of tubular dilatation simulated by axial stretch of RTE cells. Renal interstitial fluid will be collected from kidneys from rats with UUO to identify the compounds increased by UUO, as well as their interstitial concentration. The effects of these compounds (in physiologic concentrations) will then be tested using in vitro cell systems. In the proposed studies, the signaling molecules will be identified and quantitated in interstitial fluid from rats subjected to UUO. The role of individual molecules will then be tested using cocultures of renal tubular cells, macrophages, and fibroblasts. It is likely that a better understanding of the crosstalk between different renal cell types will lead to new approaches to protect nephrons from cell death resulting from tubular obstruction in the developing kidney. This will lead to improved outcomes in adulthood.